The number of detected baryons in the Universe at z<0.5 is
much smaller than predicted by standard big bang
nucleosynthesis and by the detailed observation of the Lyman
alpha forest at red-shift z=2. Hydrodynamical simulations
indicate that a large fraction of the baryons today is
expected to be in a ''warm-hot'' (105–107~K)
filamentary gas, distributed in the intergalactic medium
(WHIM). It is currently believed that a significant fraction
(\~10%) of the diffuse x-ray background in the energy band
0.5-1~keV is due to thermal emission from this intergalactic
medium. The signature of the WHIM can be observed in
red-shifted (z<1) strong soft x-ray and UV emission lines
from highly ionized elements.

Using the prediction of the hydrodynamic models we simulated
the expected x-ray emission due to the WHIM and compared it
with local and high red-shift components. We then
investigated the ability of future missions (Astro-E2,
Constellation-X and MBE – a proposed dedicated SMEX mission)
to detect the X-ray emission due to the WHIM, and we
quantified the effect of the WHIM x-ray emission on current
and past experiments.

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